Process For Recycling Light Metal Parts

ABSTRACT

A process for recycling light metal parts with gas inclusions or non-metallic particles. In order to convert different waste products into high class products in a process-technically simple manner, without requiring cleaning/purifying of the used materials, that from at least one light metal part with gas inclusions and at least one light metal part with largely dense, non-metallic particles, a gas containing metal melt is produced and a metal melt is allowed to solidify at an under-pressure for at least sometime under formation of a light metal foam body.

The invention relates to a process for recycling light metal parts with gas inclusions or non-metallic particles.

The invention further has as part of its objects the use of die casting scrap material as well as a use of chips of passivating metals.

An efficient utilisation of resources is propagated nowadays not only from the point of view of environment protection but is also increasingly offered from the operational-economic point of view. High raw stock prices and disposal costs compel product manufacturers in various fields to reuse or recycle waste created within the framework of the production. For example, manufacturers of light metal components find themselves confronted with the requirement of purposefully utilising large quantities of waste from casting processes and chip-forming shaping processes.

In an optimum manner, a raw stock-saving and energy-saving utilisation of wastes yields high-value products in a few process steps. This obviously holds good also for the preparation and re-utilisation or recycling of light metal wastes. Depending on the origin of the waste, light metal wastes can have gas inclusions and/or non-metallic particles, which makes. it difficult or even impossible to integrate such wastes into a production process without complexity.

Light metal parts or wastes having gas inclusions that occur in large quantities, for example, during casting process, particularly during die casting, are not suitable for direct production of high-class light metal components on account of the mentioned gas inclusions. Therefore, in order to enable re-use even for light metal components of high class, such light metal parts are melted and the melt is degassed under overheating, so that after solidification of the melt one obtains a dense light metal that can be subsequently used e. g. in a casting process.

Just as in the case of light metal parts having gas inclusions, also in case of light metal parts having non metallic particles like magnesium chips, a cumbersome cleaning/purification of the material is necessary before the light metal material can be used for producing high-class components. Here one proceeds in such a way that the impure material is melted, the disturbing particles are removed from the melt and the melt is subsequently allowed to solidify. At a high degree of purity, a preliminary material thus obtained can be subsequently used for production of light metal parts of high quality.

Although it is possible to produce high-class preliminary materials for light metal parts from light metal parts with gas inclusions or non-metallic impurities, complicated cleaning/purification operations are necessary for this.

Based on these factors, it is the problem of this invention to present a process for preparing light metal parts with gas inclusions or non-metallic particles, whereby such waste products can be transformed in a simple process-technical manner into high class products, without any requirement of cleaning/purifying the material used.

This problem is solved by a process of the type mentioned above, characterized therein that from at least one light metal part with gas inclusions and at least one rather dense light metal part with non-metallic particles, a gas-containing metal melt is produced and the metal melt is allowed to solidify at vacuum for at least sometime under formation of a light metal foam body.

The advantages targeted by the invention can mainly be seen therein, that impure or contaminated light metal parts can be directly transformed into light-weight metal foam bodies in a simple process, without any necessity of cleaning/purifying the light metal parts used. Advantageously, in the process according to the invention, light metal parts with gas inclusions as well as rather dense, non-metallic particles are used, so that different contaminated light metal parts or types of light metal wastes can be re-used or recycled at the same time. The obtained metal foam bodies are suitable as energy-absorbing and sound-absorbing components for thermal insulation or as reinforcing elements in the automobile industry. Thus low class waste products can be transformed directly into high class metal foam bodies with multiple uses according to the invention.

With regard to the mechanism of metal foam formation, it is assumed that gas inclusions or non-metallic particles introduced into the metal melt by the respective light metal parts act together: through the gas inclusions of a first light metal part, the gas required for metal foam formation is brought in, whereby during melting of the light metal part the gas inclusions are retained in their form; similarly, the non-metallic particles present in the metal melt deposit themselves on the surface of the gas inclusions or gas bubbles due to energetic reasons, whereby these get stabilised and a coalescence of the same is prevented. Moreover, the non-metallic particles increase the viscosity of the metal melt and thus reduce a mobility of the gas inclusions or gas bubbles in the metal melt. This also reduces the tendency of the gas inclusions to rise up to the surface of the melt and exit there. Applying vacuum ultimately causes foaming of the gas-containing metal melt under formation of the light metal foam body.

As far as a comparison to traditional processes for producing metal foam bodies can be drawn, the special feature of the process according to the invention is that, neither are special measures required to introduce a gas nor is it necessary to carry out a complicated separate production of expanding agent components and light metal powder components, as is the case in melt-metallurgical or powder-metallurgical processes.

Basically, in a process according to the invention, light metal parts from different light metals can be used. However, it is favourable if light metal parts from the same metal or the same alloy are used. These light metals then melt within a small temperature interval, which makes conducting and control of the process simpler.

Furthermore, it is advantageous if a die casting scrap is used as light metal part with gas inclusions. Die casting scrap, for example in the form of so-called overflows that occur during die casting process, can have a share of gas inclusions of more than approx. 10 vol. %, which has proved to be useful with respect to high introduction of gas in the metal melt.

It is particularly advantageous if the die casting scrap part consists of magnesium or magnesium alloy. Parts of these materials are rendered passive on the surface, which leads to formation of magnesium particles or magnesium films. If such parts are subsequently used in a process according to the invention, then stabilising magnesium oxide particles can be additionally fed. Besides, even gas inclusions in the interior portions are rendered passive at the same time; this contributes positively to the stability of gas inclusions or gas bubbles in the melt.

Similarly, a part made of magnesium or a magnesium alloy can be used as dense light metal part, so that stabilisation of gas bubbles in the metal melt is mainly caused by magnesium oxide particles.

It is advantageous if the non-metallic particles are essentially oxide particles, as these particles behave uniformly inert with respect to reactions with light metal melts. Contrary to carbides, for example, in case of presence of such particles in a material used, it can be assumed that these particles are present almost unchanged in the formed metal foam body. Thereby even properties of the formed light metal foam body can be reliably influenced. Thus, on this basis, the content of non-metallic particles in the produced metal foam body can be controlled. If a particle content in the used material is known, then one merely has to select a weight ratio of light metal parts with gas inclusions to light metal parts with oxide particles according to the desired particle content in the metal foam body.

In the context of non-metallic particles it has been seen that it is advantageous if these have an average size of lesser than 200 μm.

Ideally, a metal melt is produced with a volume share of non-metallic particles of 2 to 10%. Above a particle content of 2% a good stabilising of gas inclusions/gas bubbles in the melt is attained; up to a particle content of 10% the gas-containing metal melts can easily be foamed up by applying vacuum: besides, increasing viscosity would make foaming of the metal melts difficult.

The metal melt is foamed and transformed into a light metal foam body by allowing it to solidify at an vacuum of 10 to 400 mbar, particularly 50 to 200 mbar.

It is also possible to additionally feed gas into the metal melt over the melt surface by application of gas pressure, in order to support subsequent foaming. However, process-technically and apparatus-wise it is particularly simple if the metal melt is produced at atmospheric pressure. In this case, during the entire process the atmospheric pressure is not exceeded.

After creation, the metal melt should not be overheated by more than 20° C. The viscosity of metal melt decreases with increasing temperature, which basically favours a mobility of gas bubbles and hence degassing. It would therefore be ideal to keep overheating of the metal melt controlled and low.

It is preferable if the vacuum a applied on reaching a temperature in the range of 5° C. above to 5° C. below the solidification temperature or the solidification interval of the metal melt. In this temperature range the fluid phase of the metal has a high viscosity, which proves to be favourable with respect to the structural stability of the formed metal foam.

With respect to the structural stability of the formed metal foam, it is similarly preferred if the vacuum that causes foaming of the metal melt is maintained till complete solidification of the metal melt. At the same time, the metal melt can be cooled during solidification, in order to discharge solidification heat that is released and at the same time to freeze an inner structure of the formed light metal foam body.

In order to design a light metal foam body with a particular shape, the metal melt can be transferred to a container giving shape to the light metal foam body before applying vacuum.

In a further embodiment of the invention, during formation of the light metal foam body, parts of the same or metal foam can be brought in contact with a metal body. In that case, the formed metal foam is similarly bonded in a process step with a metal body by a metal bond. This allows an extremely simple production of compound parts.

In another embodiment of the invention the metal melt is allowed to solidify in an essentially closed container that limits spatial expansion of the light metal foam body that gets formed. In this case, on the one hand, the volume of the ready light metal foam body is pre-given; on the other hand, even the mass introduced into the container can be selected or is fixed and consequently the density is also fixed. In other words, the invention allows specific adjustment of the density of the produced light metal foam body.

The process according to the invention can be used for producing several identic light metal foam bodies, if the metal melt is introduced into the container portion-wise and allowed to solidify there.

Using die casting scrap for producing metal foam bodies has proved to be extremely useful. Chips of passivating metals can be considered as second light metal component. Such chips that occur in large quantities as waste product in lathe machining of aluminium or magnesium work pieces are particularly advantageous to the extent that, on account of a surface having oxide particles rendered passive, they are best suited for introducing stabilising particles. Besides, for low volumes these chips have a large surface, so that already a small quantity of such chips is sufficient to enable foaming of a metal melt from die casting scrap and formation of a stable metal foam.

The invention is further described below on the basis of examples. The following are shown:

FIG. 1: A cut up magnesium foam body;

FIG. 2: A micrograph of the magnesium foam body from FIG. 1 in approx. 25-times enlargement;

FIG. 3: A micrograph of the magnesium foam body from FIG. 1 in approx. 90-times enlargement;

FIG. 4: Stress and compression diagrams of metal foam bodies with a density of a) 0.56 gm/cm³, b) 0.41 gm/cm³, c) 0.36 gm/cm³ and d) 0.23 gm/cm³;

FIG. 5 a: Metal foam body of alloy AZ 91, where vacuum was applied after reaching 580° C.;

FIG. 5 b: Metal foam body of alloy AZ 91, where vacuum was applied after reaching 600° C.;

FIG. 5 c: Metal foam body of alloy AZ 91, where vacuum was applied after reaching 620° C.;

FIG. 6 a: Magnesium foam body that was produced in an open container;

FIG. 6 b: Magnesium foam body that was produced in a closed container;

FIG. 7: A device for foaming hollow bodies;

FIG. 8: A composite part consisting of an aluminium tube with a magnesium foam core.

In a recycling process according to invention, die casting scrap parts made of magnesium alloys AZ 91 and AM 50 were used for producing light metal foam bodies. The die casting scrap parts revealed pore inclusions with a volume share of respectively approx. 20%. The chemical composition of the used scrap parts is given in table 1.

TABLE 1 Chemical composition of alloys AZ 91 and AM 50 Alloy Al [%] Be [%] Cu [%] Fe [%] Mn [%] Ni [%] Si [%] Zn[%] AZ91 9.2 <0.001 <0.005 0.007 0.19 <0.001 0.03 0.75 AM50 4.7 <0.001 <0.005 0.002 0.29 <0.001 0.02 <0.01

Die casting scrap parts made of AZ 91 or AM 50 were melted together with magnesium chips of the same alloy in the weight ratio of die casting scrap parts: chips=7:1 in an open crucible under atmospheric pressure. The thus produced metal melts were transferred into steel moulds. Subsequently the filled steel crucibles were placed in a vacuum chamber and subjected to a vacuum of 80 mbar at a temperature of 600° C. (AZ 91) or 630° C. (AM 50), whereby the metal melts foamed and light metal foam bodies got formed.

A thus produced light metal foam body is shown in section in FIG. 1. As one can see, the light metal foam body has several pores and a dense outer surface. Apart from the pores seen in FIG. 1 that have a diameter of a few millimetres, there are further pores with smaller diameters as one can see from the micrographs in FIG. 2 and FIG. 3. The non-metallic particles have an average size of under than 200 μm.

The pores structure shown more clearly in FIGS. 1 to 3 result in an energy absorption behaviour that makes light metal foam bodies produced from recycling material usable for many applications in vehicle manufacturing.

In FIG. 4 stress/compression diagrams in compression tests on parallelepipeds (5×5×3 cm³) are depicted for light metal foam bodies made of alloy AZ 91 and different densities. These compression curves for parallelepipeds having a density of a) 0.56 gm/cm³, b) 0.41 gm/cm³, c) 0.36 gm/cm³ and d) 0.23 gm/cm³ prove that recycled light metal foam bodies in the compression test show a pronounced plateau after a short linear ascension. In this plateau region, the so-called deformation stress, the light metal foam bodies are effective as energy absorbers. The deformation stresses attained in the reforming light metal foam bodies lie in the region of those light metal foam bodies that are produced in the traditional way, e.g. by powder-metallurgical or melt-metallurgical method.

In FIGS. 5 a to 5 c the effect of temperature at the beginning of vacuum onto the shape of a light metal foam body made of AZ 91 alloy is shown. FIGS. 5 a and 5 b show in comparison, that at 580° C. the foam formation is partially suppressed, which is due to very high viscosity of the material to be foamed at this temperature. At 600° C., which somewhat corresponds to the melting point of AZ 91, the foam formation process is optimized with respect to a complete expansion of the foam body. Higher temperatures like 620° C. (FIG. 5 c) result in reduction of viscosity of the metal melt. Consequently the formed metal foam body can partially collapse because a wall structure of the metal foam is not sufficiently stable.

Apart from the selection of a foaming temperature or a temperature at which vacuum is first applied, even specific cooling conditions promote homogeneous formation of light metal foam bodies: a uniform cooling on all sides is advantageous and can be achieved by insulating the container in which foaming is carried out.

FIG. 6 a and 6 b juxtapose formations of light foam bodies under same condition, especially same temperatures, vacuums and same used mass of metal melts, in an open container (FIG. 6 a) and in a closed container (FIG. 6 b). If a container is open, then the formed metal foam can expand, unhindered and one obtains a light metal foam body as shown in FIG. 6 a. However, if a container is closed in such a way that even though a vacuum can be generated within it, an expansion of the formed metal foam body is restricted; thus the shape of the container defines the shape of metal foam body. In such a case, one can obtain a porous light metal foam body of a desired shape and with an almost dense surface.

On basis of FIG. 7 and 8 it is further explained how a process according to the invention can be applied for producing compound parts. FIG. 7 shows an vacuum chamber 1 with a gas inlet 6 and a gas outlet 7 that are mounted on a cover 8 of the vacuum chamber 1. The cover 8 can be removed from a side wall 5 of the vacuum chamber 1, so that a steel mould 2 can be introduced into the vacuum chamber 1. After introducing the steel mould 2 this serves the purpose of taking up a gas-containing metal melt 4. It is obvious that it is similarly possible to first put the metal melt 4 into the steel mould 2 and subsequently the steel mould 2 into the vacuum chamber 1.

Apart from the steel mould 2, a hollow body 3 is introduced into the vacuum chamber 1. The hollow body 3 is thereby placed in such a way, that the metal foam formed from the metal melt 4 can come in contact with it. As shown in the example in FIG. 7, this can take place in such a way that a hollow body 3 in the form of a tube is placed in a conically shaped steel mould 2 just above a melt surface.

The hollow body 3 can be of any metal or alloy. For example, combinations with light metal foam bodies of magnesium or magnesium alloys, steel profiles or aluminium profiles are suitable. It is particularly advantageous if the hollow body 3 to be foamed out is made of a metal or an alloy having a melting point/melting interval that lies appx. in the range of the metal melt. Then contact with the metal-foam at or just below the melting temperature effects melting of the hollow body 3 at the contact face, which promotes solid metal bonding of light metal foams and hollow body 3. It is also advantageous to pre-treat the surface of the used hollow body 3 in the desired contact region, e.g. by removing passivating films/layers in order to achieve a full-faced metal bonding as far as possible.

FIG. 8 shows a section through a compound part produced according to the arrangement shown in FIG. 7. An outer region of the compound part is formed by the used hollow profile 3 and the inner region from light metal foam.

It is obvious to the expert that not only is it possible to foam out hollow profiles, but that similarly with compound parts produced with the same quality also re-foaming of metal bodies and combinations of foaming out/re-foaming are possible. 

1-20. (canceled)
 21. A process for producing metal foamed bodies from scrap metal parts comprising the steps of: (1) melting scrap metal parts to form a metal melt; (2) applying a vacuum of 10 to 400 mbar to the metal melt; and (3) solidifying the metal melt under the applied vacuum to produce a foamed metal body.
 22. A process according to claim 21, wherein the vacuum is between 50 to 200 mbar.
 23. A process according to claim 21, including feeding a gas to the metal melt under vacuum.
 24. A process according to claim 21, wherein heating the metal melt to a temperature in the range of 5° C. above to 5° C. below the solidification temperature of the metal in the melt prior to applying the vacuum.
 25. A process according to claim 21, wherein the scrap metal parts are selected from the group consisting of metal parts with gas inclusions, metal parts with non-metallic particles, and mixtures thereof.
 26. A process according to claim 25, wherein the metal parts have non-metallic particle content of between 2 to 10% by vol.
 27. A process according to claim 21, wherein the metal parts have non-metallic particles of a size of less than 200 μm.
 28. A process according to claim 21, wherein the metal parts are made of the same metal or the same metal alloy.
 29. A process according to claim 21, wherein a die casting scrap part with gas inclusions is used as the metal part.
 30. A process according to claim 29, wherein the die casting scrap part is made of magnesium or a magnesium alloy.
 31. A process according to claim 25, wherein the non-metallic particles are oxide particles.
 32. A process according to claim 31, wherein the non-metallic particles have an average size of less than 200 μm.
 33. A process according to claim 21, including, before applying vacuum, the metal melt is transferred into a container having the shape of the light metal foam body to be produced. 